1. Field of the Invention
The present invention relates to a structural body and an X-ray Talbot interferometer that includes the structural body.
2. Description of the Related Art
Diffractive gratings that include a structural body having a periodic structure are used as optical elements in various apparatuses. In particular, X-ray optical elements containing gold, which has high X-ray absorptivity, are used in non-destructive inspection of substances using X-rays and in the medical field.
An application of X-ray optical elements containing gold is the shield grating of Talbot interferometers, the latter of which acquire information on a subject utilizing X-ray Talbot interference.
X-ray Talbot interferometry will be briefly described below. X-ray Talbot interferometry is one of a number of imaging methods (X-ray phase imaging methods) utilizing X-ray phase contrast.
In general X-ray Talbot interferometers, a spatially coherent X-ray is diffracted by a diffractive grating and forms an interference pattern. A moiré pattern is formed by placing a shield grating configured periodically to block an X-ray at a position where the interference pattern is formed. The moiré pattern is detected with a detector. A subject disposed between an X-ray source and the shield grating changes the moiré pattern. Information (in general, phase image information, differential phase contrast image information, scattering image information, or absorption image information) on the subject can be obtained from this change in the moiré pattern.
General shield gratings for use in Talbot interferometry have a structure in which X-ray transmission portions (hereinafter also referred to simply as transmission portions) and X-ray shield portions (hereinafter also referred to simply as shield portions) are periodically arranged. The X-ray shield portion is often composed of gold, which has high X-ray absorptivity. Even using a highly-absorbent material such as gold in a shield portion, the shield portion structure has a high aspect ratio based on the relationship between the thickness required for X-ray shielding and the period of the interference pattern impinging on the shield grating (the aspect ratio is the ratio (h/w) of the height or depth h to the width w of the shield portion). Hereinbelow, a shield grating composed of gold is considered to be a type of gold layer. Flat shield gratings are useful when parallel light (e.g. a parallel X-ray) is used, for example, in synchrotron radiation facilities. However, like laboratory X-ray tubes, in the case of imaging using spherical waves emitted by an X-ray source, which is a point light source for emitting divergent light (a divergent X-ray), the X-ray propagation direction deviates from the height direction of the shield portion with increasing distance from the optical axis (the X-ray axis). This also blocks an X-ray that should be transmitted through the shield grating, resulting in an insufficient X-ray transmission contrast or a low X-ray dosage that reaches a detector. These may result in increased noise in the information on the subject or no information on the subject in a peripheral region that is distant from the optical axis.
Japanese Patent Laid-Open No. 2007-206075 (US counterpart: 2007/0183583) discloses a method for conforming the height direction of the shield portion to the X-ray propagation direction by placing a shield grating in a vacuum chamber and two-dimensionally bending the shield grating by the action of a differential pressure, thereby bending the shield grating along a spherical surface to match the wavefront of the divergent X-ray.
In order to retain the curved shape of the shield grating using the method according to Japanese Patent Laid-Open No. 2007-206075, however, the differential pressure must be continuously applied to the shield grating. Thus, it may be difficult to retain the curved shape of the shield grating. Gold layers other than gratings suffer the same problems.